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Monday, September 18, 2017

Wyoming's Exotic and Enigmatic Phonolite

Folks, it’s phonolite!

In the late 1970s, I worked as a ranger naturalist at Devils Tower National Monument in northeast Wyoming. During my stints at the desk in the Visitor Center, I heard the Admatic Automatic Slide Projector’s short program about the Tower over and over. One phrase is especially memorable: “phonolite porphyry, named for the ringing sound the rock makes makes when struck.”

I thought it would be fun to demonstrate this for visitors, but I was never able to produce a ringing sound by striking the Tower rock. Interestingly, the Park Service now says that “phonolite (fō'nə-līt') refers to the mineral composition of the rock.” However “porphyry” hasn't changed; it still refers to rock texture—visible crystals (in this case feldspar) in a fine-grained matrix.

Northeast face of Devils Tower.

Futilely striking Tower rock.

Devils Tower phonolite porphyry.

Phonolite is an uncommon rock—“exotic and enigmatic.” Yet I encountered it again just recently, in the Rattlesnake Hills of central Wyoming only 175 miles southwest of Devils Tower. In both places the phonolite is estimated to be about 40 million years old, emplaced during the Eocene Epoch.

Phonolite is igneous—formed from magma. It’s often referred to as volcanic (e.g., here and here), which means it's extrusive—magma reached the surface. And yet the Devils Tower and Rattlesnake Hills phonolite bodies are considered intrusions that formed below the surface, though probably not deep. The phonolite in the Rattlesnake Hills has been called “subvolcanic.”

Devils Tower is just one of a collection of igneous intrusions of Eocene age in the northern Black Hills. Rock types include members of the phonolite-trachyte-latite series. They probably were emplaced late in the uplift of the Hills (part of the Laramide Orogeny, the building of the Rocky Mountains). The intrusions were later exposed by erosion, and now form prominent landscape features: Devils Tower, the Little Missouri Buttes, Sundance Mountain, Inyan Kara Mountain, Crow Peak, and more.

Sundance Mountain; note columnar jointing (click on image to view).

During cooling and contraction of the intruded magma, columnar jointing sometimes formed. As Evelyn Mervine has said, “Columnar jointing is always a joy to observe in rocks in the field. Stumbling upon perfectly geometric columns of rock can only be described as magical.”Many northern Black Hills intrusions include columnar jointing, but never as extensively and dramatically as at Devils Tower—not even close! There the columns are as much as six feet across and 600 feet tall.

South face of Devils Tower.

Like the Black Hills, the Rattlesnake Hills were created during the Laramide Orogeny. They also include Eocene igneous intrusions. Composition is similar to those of the Black Hills, but phonolite is more common, found in 21 intrusions in the central core of the range (Hoch and Frost 1993). Columnar jointing occurs here too.

The same rare rocks of about the same age in close proximitydemands an explanation! Are Black Hills and Rattlesnake Hills intrusions related? Sadly, it appears no one knows. Their origins are “poorly understood” and the phonolite remains enigmatic. Hoch and Frost (1993) think that Laramide structural features (faults, fractures) “enabled the alkaline and subalkaline magmas to ascend to shallow crustal levels.” But why would there be exotic magmas here?

Geochemical analyses indicated that the magmas must have involved some mixing. But Archean country rocks of the Wyoming craton—our several-billion-year-old basement rocks—are poorly represented in the mix. Hoch and Frost wrote that the magma “may reflect the composition of subcontinental mantle that has been isolated and chemically modified during the last 3.5 billion years.”

In other words, magma made from deep 3.5-billion-year-old rocks moved up through fractures 40 million years ago, and solidified before reaching the surface. Then starting around five million years ago, erosion carried away the overlying softer rocks, revealing the intrusions. If correct, this will be a really cool story! But more work needs to be done. The phonolites are not eager to give up their secrets.

North face of Devils Tower. “It is somewhat of a geological puzzle, standing alone as it does, and rising directly out of a country entirely made up of sedimentary rock.” Thomas Moran, 1894